A magnetic recording medium is conventionally prepared by coating a binder and a ferromagnetic material which is plate-shaped and which has an easy axis of magnetization that is perpendicular or vertical to the surface of the plate on a non-magnetic support. The ferromagnetic material can include barium ferrite, of which barium or iron can be partially substituted with a metal such as Ca, Sr, Pb, Co or Ni, or MnBi of which Mn or Bi can be substituted with a metal such as Se.
It is well known that after coating a magnetic coating composition and before drying the coating layer, magnetic particles are subjected to orientation in the magnetic field toward the recording direction to improve the electromagnetic characteristics in this direction. For example, this method has been employed for preparing a magnetic recording tape using an acicular .gamma.-Fe.sub.2 O.sub.3, as is disclosed in Japanese patent publication Nos. 5350/65, 23624/65, 23626/65, 2065/66 and 21251/68.
It is clear, however, that the orientation degree is markedly insufficient as evidenced when observating the orientation degree of the magnetic particles using an electromicroscope or X-ray diffraction.
Yasushi Hoshino, "New Science for Industrial Materials, Composite Materials IV" published by Kinbara Shuppan in 1969 on page 63 discloses that CrO.sub.2 is more easily oriented than other ferromagnetic materials, but that even CrO.sub.2 is incompletely oriented with high speed coating (not less than 20 m/sec.).
It is believed that the reason for incomplete magnetic orientation is the high thixotropic property of various magnetic coating materials.
It is disclosed in Japanese patent publication No. 856/81 that a magnetic layer is provided on a support and before the magnetic layer is dried, a flexible sheet is contacted with the surface of the magnetic layer while applying a magnetic field thereon to improve the degree of orientation.
Further, it is taught in "Nikkei Electronics", pages 100 to 111 (Aug. 7, 1978) and in IEEE Transactions on Magnetics" Vol. MAG-15, No. 6, pages 1561 to 1563 (November, 1979) that a perpendicular magnetic recording system is excellent in a high density magnetic recording system.
A method for smoothening an uneven surface containing pin-holes, longitudinal lines or streaks, irregular coating thickness, etc. by contacting the flexible sheet (hereinafter "sheet") with the surface of the not yet dried or solidified coating layer, is employed in the industry for coating highly viscous solutions as disclosed in Japanese patent publication No. 96469/72 and 53631/74.
In employing this smoothening method to prepare magnetic recording tapes, the inventors of the present invention have found that the orientation of the magnetic particles is markedly improved by applying magnetic field to produce a magnetic orientation at the smoothening area. It has been also found that the orientation of magnetic particles is greater than in any instances where only the magnetic orientation is conducted, where the smoothening method is applied after magnetic orientation is conducted, where magnetic orientation is conducted after the smoothening method is applied, and where only the smoothening method is applied and magnetic orientation is not conducted.
The present invention is more effective because, when the smoothening method is carried out, a stronger shearing force is applied parallel to the surface of the undried coating layer and the coating layer becomes markedly fluid which results in temporarily lowering thixotropy of the coating solution on the web.
The present invention is an improvement of the method as disclosed in Japanese patent publication No. 856/81 in which a plate-shaped magnetic material having magnetic anisotropy perpendicular to the plate is employed, an easy axis of magnetization is subjected to orientation perpendicular to a surface of a support and perpendicular to the magnetic component is used to prepare a magnetic recording medium suitable for a magnetic recording system.